1. Field of the Invention
This invention relates to a semiconductor device composed of high voltage transistors which is applied to, for example, the peripheral circuitry of an electrically rewritable semiconductor memory device.
2. Description of the Related Art
One of the nonvolatile semiconductor memory devices is a NAND type EEPROM (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-324400). In a NAND type EEPROM, when a NAND cell is written into, a high positive write voltage Vpgm ranging from 15V to 40V is applied to a word line serving as a data select line and 0V or a voltage Vdd is applied to a bit line serving as a data transfer line. Vdd is a voltage in the range of 1V to 5V. Vpgm is supplied from a driving signal line for driving a word line. A word line connect transistor is provided between a word line driving signal line and a word line. The word line connect transistor switches between the transfer and untransfer of Vpgm to a word line with a program selected/unselected block. Vdd is supplied by a driving signal line which drives a bit line. A bit line connect transistor is provided between a bit line driving line and a bit line. The bit line connect transistor is controlled so as to transfer Vdd to a bit line connected to a cell not to be programmed and 0V to a bit line connected to a cell to be programmed.
When a NAND cell is erased from, 0V is applied to the word line of the selected erase block and a high positive voltage Vera in the range of 15V to 30V is applied to the word lines of the unselected erase block, that is, the word lines of the blocks not to be erased from. At this time, the bit lines are kept in the floating state. As the potential of a well in which a NAND cell has been formed rises from 0V to Vera, the high voltage Vera is applied to the bit line through capacity coupling. However, turning off the bit line connect transistor prevents the high voltage from being applied to the sense amplifier. Accordingly, the circuitry on the sense amplifier side does not need a high-voltage transistor whose breakdown voltage is equal to or higher than 15V and therefore can use a transistor which is not needed to consider voltage-proof design, for example, thin gate insulating film. This makes it possible to reduce the area of the circuitry on the sense amplifier side remarkably as compared with a case where the circuitry on the sense amplifier side is composed of high voltage transistors.
As described above, when a memory cell is written into or erased from, the word line connect transistor and the bit line connect transistor require Vpgm or Vera equal to or higher than 15V to be cut off. Therefore, a transistor whose junction breakdown voltage is equal to or higher than 15V is required. However, an attempt to raise the breakdown voltage of the transistor requires the gate length, the distance between the gate electrode and the contact, and the like to be increased. This causes the problem of making the size of the transistor larger and increasing the area of the chip. Accordingly, a semiconductor device with high voltage-proof and small transistors, which decreases the area occupied by the region where transistors are formed, has been required.